Hi Poppy
Glad to see that you're back. I had a chemical pregancy too at 4.5 weeks and it's like once you start to get your mind around it, it's all wisked away like it never happened. My doctor said that I could start trying again straight away. We did and I ovulated right on time. He said that "if it was 6 weeks or more that it might be different, but (even though it doesn't sound very nice) your body sees a miscarriage this early as like a period." If you want to start ttc straight away I would use OPK's, that way you would be able to avoid the confusion with dating the pregnancy if you did fall straight away and that's the only reason they advise you to wait a month. The other thing is for calculation purposes, you count the first day of proper miscarriage bleeding as CD1. I didn't conceive again straight away, but it's now 3rd try since and I think I got a faint bfp on ultra earlies this morning, so I'm sure it wont be long for you either - My Doc also said that it's like the body sometimes needs a trial run and that it could happen again really quickly as It had started gearing up for it once before.
Anyway, whatever you decide, it should be what feels right for you. I hope you get a BFP again soon. Good luck!